Bone fractures may occur in straight bones, such as the femur, or in curved bones, such as pelvic bones. Repairing a bone fracture generally involves two steps: fracture reduction and fracture fixation. Reduction is the step of reducing the fracture by minimizing the distance between the bone fragments and aligning the bones anatomically to minimize deformity after healing. Both surgical and nonsurgical reduction methods exist. Fixation is the step of holding the bone fracture fragments mechanically stable and in close proximity to each other to promote bone healing which may take several weeks or more, depending on the type of fracture, type of bone and the general health of the patient suffering the injury.
Fixing bone fracture fragments in a mechanically stable manner to eliminate motion across the fracture site also minimizes pain when patients apply weight across the fracture during everyday activities like sitting or walking. Fixation of bone fractures may be accomplished by either internal or external fixation. Internal fixation is defined by mechanically fixing the bone fracture fragments with implanted devices. Examples of internal fixation include bone screws inserted within the bone across the fracture site and bone plates which are applied to the surface of the bone across the fracture site. Bone plates are typically attached to healthy bone using two or more bone screws.
External fixation is defined by methods and devices which mechanically fix the bone fracture fragments with devices or methods external to the body. The traditional use of a splint or cast are examples of external fixation of a fractured bone. An example of an invasive external fixation device uses long screws that are inserted into bone on each side of the fracture. In pelvic fracture work the use of external skeletal fixation is common and involves placing long threaded pins into the iliac bones and then connecting them with an external frame. These screws are connected to a frame which is located outside the body.
Invasiveness of both fracture reduction and fixation steps varies depending on the devices and/or methods used. Invasive open reduction typically involves surgically dissecting to allow access the bone fracture. Dissection is performed through the skin, fat, and muscle layers, while avoiding injury to adjacent structures such as nerves, major blood vessels, and organs. Once dissection has been completed, the fracture may be reduced prior to definitive fixation and provisionally held using surgical clamps or other methods. Non-invasive closed reduction is typically performed by applying force to the patient's skin surface at different locations and/or to apply traction to a leg, to reduce the fracture. Minimally invasive reduction techniques minimize the surgical dissection area by reducing the size of the surgical wound and by directly pushing on the bone with various long handled tools through the minimal surgical wound. Invasive open fixation typically involves surgically dissecting to allow access to sufficient areas of healthy bone so that fixation devices such as surgical plates can be attached directly to the bone surface to fix the fracture site. Minimally invasive closed fixation typically involves insertion of a device such as a bone screw or intramedullary rod (or nail) within the bone through a small incision in the skin, fat, and muscle layers.
Minimally invasive reduction and fixation are typically used to repair long bone injuries such as the femur. One example is an intramedullary rod, also known as an intramedullary nail (IM nail), inter-locking nail or Küntschner nail. Intramedullary nails in the femur and tibia are load sharing devices and can well resist large bending and shearing forces, thereby allowing patients to leave hospital and manage with crutches in a short time.
The mechanical strength of bone fixation is determined by both the strength of the implant and strength of the implant's attachment to healthy bone. The mechanical forces applied across the fracture during the healing process can include shear, compression, tension (tensile), torsion, static loading and dynamic loading. In bones with complex curvature such as bones of the pelvis (FIG. 1) or of the spine, straight intramedullary fixation devices have limitations. Bone curvature limits the mechanical strength of attaching a straight intramedullary fixation device within healthy bone tissue. In pelvic and acetabular fracture fixation, and example of a straight intramedullary device is a commonly used cannulated bone screw. These screws must be limited in length and diameter because they are a straight device in a curved tunnel. If too long they will penetrate the bone and could injure important soft tissues. However, such screws may not offer secure fixation due to their low tensile pull out forces in cortical cancellous and/or osteoporotic bone during the healing process. Also, the diameter of the straight intramedullary screw, when in a curved bone, is significantly smaller than the thickness of the cancellous bone layer between the two outer cortical bone layers. Since the cancellous bone is significantly weaker than cortical bone (and can have significantly compromised strength in the case of osteoporotic bone) straight intramedullary screws may allow for the bone fragments to move relative to each other due to inadequate vertical shear holding force of cancellous bone. Plates normally act, mechanically, as tension band plates, neutralization plates or compression plates. Often a single plate will perform more than one of these mechanical functions, but since the plates are attached to the bone, the use of plates requires invasive open surgery to expose the bone. The plates are inherently weak because they have to be designed to be thin and have notches in them so that they can be bent to fit the curves of the pelvis. Invasive open surgery can result in increased blood loss, increased risk of infection and increased healing time compared to minimally invasive methods.